1. Field of the Invention
The present invention relates to a method for manufacture of a multilayer ceramic substrate which is applicable for use in mobile communication terminals such as portable phones, and also to a multilayer ceramic substrate.
2. Description of Related Art
With the need of miniaturizing recently spread mobile communication equipment, such as portable phones, and portable communication terminals, the size reduction and performance increase of high-frequency circuit parts for use therein have been pursued.
In high-frequency circuit substrates, a conventional module including a printed substrate and a capacitor or inductor mounted on its surface has been increasingly replaced by a smaller size module consisting of multilayers of dielectric ceramic substrates each patterned with a capacitor or inductor circuit.
The multilayer ceramic substrate is generally fabricated by providing multilayers of green sheets, such as glass-ceramics, each carrying a predetermined circuit pattern formed by screen printing or the like, and firing them at a temperature of about 900° C. A popular material used for the circuit pattern is silver (Ag) which is highly conductive and air-firable.
However, due to a higher tendency of silver to exhibit a migration phenomenon, short circuit defect often occurs between silver wiring electrodes separated by a dielectric layer, which has been a problem.
Japanese Patent Laying-Open No. Hei 11-49531 describes that inclusion of CuO in a glass component of a glass-ceramic for use in a dielectric layer suppresses ionization of Ag during a firing process and thus restrains diffusion of Ag into the glass-ceramic.
In Japanese Patent Laying-Open No. Hei 8-288643, a surface roughness Ra of an insulating base, to which a wiring conductor is adhered, is specified as being in the range of 0.2–0.7 μm. This is contemplated to strengthen a joint between the insulating base and the wiring conductor in a three-dimensional fashion.
Japanese Patent Laying-Open No. 2001-339166 describes a wiring board which includes an insulating base composed of a glass-ceramic and a wired circuit layer formed on a surface or internally of the insulating base and proposes that a surface of the wired circuit layer on the insulating base, opposite to its surface in contact with the insulating base, should have a surface roughness Rz of 1–8 μm. This is described to permit a constraining sheet to constrain the wiring board more evenly so that shrinkage thereof during a firing process is restrained more effectively.